The present invention relates generally to radio-frequency (RF) communication systems and, more particularly, to such systems having a high-temperature superconducting (HTS) filter.
Radio frequency (RF) filters have been used with cellular base stations and other telecommunications equipment for some time. Such filters are conventionally used in a receiver front-end to filter out noise and other unwanted signals that would harm components of the receiver in the base station. For example, bandpass filters are conventionally used to filter out or block RF signals in all but one or more predefined bands. With the recent dramatic rise in wireless communications, such filtering should provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals) in an increasingly hostile frequency spectrum.
The relatively recent advancements in superconducting technology have given rise to a new type of RF filter, namely, the high-temperature superconducting (HTS) filter. HTS filters contain components which are superconductors at or above the liquid nitrogen temperature of 77 K. Such filters provide greatly enhanced performance in terms of both sensitivity and selectability as compared to conventional filters. However, since known HTS materials are only superconductive at relatively low temperatures (e.g., approximately 90 K or lower), and are relatively poor conductors at ambient temperatures, such superconducting filters require accompanying cooling systems to ensure the filters are maintained at the proper temperature during use.
The cooling system typically includes a cryorefrigerator, which, in turn, has a compressor for maintaining a supply of pressurized coolant and a heat exchanger or cold head to remove heat from the devices being cooled. In addition, the cooling system must minimize the amount of heat transfer from the environment by enclosing the HTS filter and other cooled devices in a cryostat. The cryostat is then often evacuated of any gaseous material to reduce convection heating.
With the aforementioned dependency on a cooling system, the reliability of traditional HTS-based receiver front-ends has been tied to the reliability of the power source. Specifically, if the power source (e.g., a commercial power distribution system) fails (e.g., a black out, a brown out, etc.) for any substantial length of time, the cooling system would likewise fail and, when the corresponding HTS components warm sufficiently to prevent superconduction, so too would the filters.
To prevent systems serviced by such filters from failing during these power outages, additional circuitry in the form of RF bypass circuitry has been used to switch out the failed filter until a suitably cooled environment was returned.
As shown in FIG. 1, in a typical HTS front-end receiver indicated generally at 10, an incoming RF signal collected by an antenna (not shown) is provided via low-loss cabling 12 to a bypass switch 14 to determine whether the RF signal will be filtered via an HTS path or a conventional path. The state of the bypass switch 14 is controlled by a control signal carried by lines 16A and 16B that may be developed by a controller (not shown) associated with the receiver that monitors the performance of a cooling system indicated generally at 18. If the cooling system 18 is performing adequately, the RF signal is passed via low-loss cabling 20 to a cryostat 22 housing the components of the HTS path. The cryostat 22 includes a port (not shown) and a coupling mechanism (not shown) for connecting the cabling 20 to an HTS filter 24 disposed inside the cryostat 22. Typically, the port and the coupling mechanism are designed to ensure both a good vacuum seal and minimal heat conduction.
The HTS filter 24 then removes signal components outside of the passband, i.e., the frequency range of the desired signal. Because the resulting filtered signal is usually very weak, the HTS filter 24 is coupled to an amplifier 26 that does not introduce a significant amount of noise (i.e., a low-noise amplifier or LNA). The amplified signal is then passed through another port (not shown) in the cryostat 22 as an output signal for the HTS path on cabling 27.
If the cooling system 18 is not functioning or otherwise not performing adequately, the bypass switch 14 provides the incoming RF signal to a conventional (i.e., non-HTS) filter 30 designed to remove signal components outside of the same passband as that of the HTS filter 24. A further bypass switch 32 determines whether the output signal on the cabling 27 or the filtered signal carried by cabling 34 coupled to the conventional filter 30 should be passed on to the remainder of the communication station. The bypass switch 32 may be controlled by a control signal provided thereto by lines 36A and 36B.
In some base stations, the bypass path has included a short-circuit between the bypass switches 14 and 32 rather than the conventional filter 30. This type of configuration has been used for installations of HTS equipment in connection with the enhancement of preexisting networks, in which case the HTS path is added onto a receiver front-end without modifying or removing a downstream conventional filter-amplifier pair.
In the event of a cooling system failure, either of the above-described bypass systems results in the need for an additional amplifier (not shown), inasmuch as the amplifier 26 disposed in the cryostat 22 is bypassed as well. Such additional circuitry adds expense and complexity to known systems.
The assignee of the present application has developed an all-temperature performance (ATP(trademark)) filter capable of both cryogenic and non-cryogenic operation. Such filters are described in commonly assigned U.S. application Ser. No. 09/158,631, and operate as a high-performance superconducting filter system when cold, and revert to the performance of a conventional filter when warm, thereby eliminating the need for a separate bypass path. However, such filters utilize thick-film technology that may not be suitable for every receiver front-end installation or certain other applications.
In accordance with one aspect of the present invention, an RF system includes a cryostat, a high-temperature superconducting filter disposed in a cold area of a chamber of the cryostat, a low-noise amplifier disposed in the chamber of the cryostat, a bypass switch coupling the high-temperature superconducting filter to the low-noise amplifier, and a conventional filter coupled to the bypass switch.
In a preferred embodiment, the low-noise amplifier is disposed in the cold region. The bypass switch may be disposed in the chamber, and the RF system may further include a further low-noise amplifier that couples the conventional filter to the bypass switch.
The chamber of the cryostat may include a warm region in which the conventional filter is disposed. The RF system preferably includes a further bypass switch coupled to the high-temperature superconducting filter and the conventional filter.
In another preferred embodiment, the bypass switch includes a mechanism for switching based on temperature, such as a bi-metallic structure.
In accordance with another aspect of the present invention, an RF system includes a conventional filter, a first low-noise amplifier coupled to the conventional filter, a cryostat, a high-temperature superconducting filter disposed in a cold region of a chamber of the cryostat, a second low-noise amplifier disposed in the chamber and coupled to the high-temperature superconducting filter, and a bypass switch having a pair of input terminals coupled to the first and second low-noise amplifiers.
The first low-noise amplifier may be disposed in the chamber of the cryostat, or in the cold region of the cryostat chamber. Similarly, the bypass switch may be disposed in the chamber, or in the cold region of the chamber.
Other features and advantages are inherent in the RF systems claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying drawings.